I've always thought it was alders on sites on the dry side for them dropping leaves in response to drought.

I see it every year.

A row of alders grows behind my back fence so I've lived with them for 5 years now. Every summer they do drop a bunch of leaves, but you're right. This year they've dropped more than usual.

Last week my backyard looked like it does in November, not July. I think summer skipped right over us this year.

Then it could be something else, related instead to cool and damp conditions.

Actually I've seen alders dropping leaves in late July most years I've been here (12 now, probably as long as you've been paying attention to trees). They drop still-green leaves in response to drought conditions. It's a common response in summer-dry climates. You'll see the ocean spray turning red this time of year too, on dry sites; same thing.

Alders like a lot of water but they often grow on very well drained sites that get bone dry by mid summer. More than water, they like nitrogen-poor, recently-disturbed soils, like roadcuts, landslides, gravel driveways and road shoulders, and winter-scoured streambanks.

If you've been drier than normal then that's probably why the alders are dropping so much already. I'm seeing it too even though we have NOT been dry at all! but it's normal leaf drop this time of year.

No Alder leaf drop here...so far. I have good soil for the most part and a lot of my 10 acres faces northeast.
Some years the leaf drop from alders is quite heavy.

Madronas drop leaves in June and July whether it's dry or not. A real mess for tidy gardeners.
Mike